1. Technical Field
The present disclosure relates to technology for conveying sheet media by a conveyor belt.
2. Related Art
Some printers, scanners and other devices that print on and read information from sheet media use a belt conveyor mechanism (belt conveyor device) that conveys the media using a conveyor belt mounted on rollers. An image forming apparatus having such a belt conveyor mechanism is taught in JP-A-2006-78906. The image forming apparatus taught in JP-A-2006-78906 transfers a toner image formed on a photoconductor to an intermediate transfer belt to forma color image on the print medium. The intermediate transfer belt is mounted on three rollers, including two pulleys and a belt conveyor drive roller. The color image is transferred from the intermediate transfer belt to paper held and conveyed between one of the two pulleys and a secondary transfer drive roller.
When conveying the medium with a belt conveyor mechanism for printing, the control unit desirably stabilizes the conveyance speed and controls media conveyance with high precision in order to assure the desired printing precision. To prevent a drop in the precision of the image transfer position from the intermediate transfer belt, JP-A-2006-78906 describes the control unit measuring the surface speed of the intermediate transfer belt with an encoder to control rotation of the belt conveyor drive roller. The encoder in JP-A-2006-78906 is disposed to the rotational shaft of a follower roller that contacts the belt surface. The encoder that measures the speed and amount of belt movement in the belt conveyor mechanism can be attached to the rotational shaft of the belt conveyor drive roller, but the speed and amount of belt movement calculated from the encoder signal in this configuration includes error introduced by the thickness of the belt and eccentricity of the encoder, for example. On the other hand, when the encoder is attached to the follower roller in contact with the belt surface to measure the speed and distance of belt travel as taught in JP-A-2006-78906, the speed and distance of belt travel are not easily affected by the belt thickness or the eccentricity of the encoder, for example. The surface speed of the belt can therefore be measured accurately, and high precision conveyance control is possible.
However, the configuration taught in JP-A-2006-78906 disposes the follower roller to contact the horizontal portion of the intermediate transfer belt, and requires providing a follower roller that is separate from the rollers defining the belt path. The parts count increases accordingly. Furthermore, while the pressure must be maintained at a level that does not produce any slipping between the intermediate transfer belt and the follower roller, the conveyance load increases if the pressure is set greater than needed. Initial adjustment of the pressure is therefore required when the follower roller is installed. In addition, because the pressure is affected by change in the belt thickness over time, regular adjustment of the pressure is also needed.